fix(ui): tighten star/block gap and border E/M/H

[tomtom-bot7]

Summary

• Group star and block in one control cluster with no gap (and slightly smaller hit targets) so they sit closer.
• Show list difficulty as bordered E / M / H chips with matching colors.

Test plan

• Problem list: star and block sit next to each other with little space between.
• E/M/H show a colored border chip; hover still shows Easy/Medium/Hard.
• Titles still have enough room; row layout looks balanced.